This invention relates to a method of mixing fluids in closed containers; more particularly the method relates to the mixing of paint and other liquids or slurries in cans which are partially filled.
The prior art as it relates to the field of mixing paints and other liquids has provided innumerable devices aimed at accomplishing effective mixing over relatively short time periods. These devices are typically intended for commercial use, as for example in a retail paint store, wherein customers purchase paint having one or more color components added to a base material, and the resulting mixture is briefly and thoroughly agitated to provide a uniform color and viscosity blend. Prior art devices and methods have had as a primary objective the thorough and effective mixing over as brief a time period as is possible, in order that sales may be rapidly accomplished and customer delay minimized. In order to accomplish this objective the apparatus for mixing has typically included a device for clamping about a paint container and for violently agitating the container for a period of thirty seconds to five minutes.
The machines designed for accomplishing the desired mixing motion have taken varied form. For example, U.S. Pat. No. 2,022,527, issued Nov. 26, 1935, discloses an oscillatory motion wherein the paint container is vertically positioned and rapidly oscillated about a horizontal axis passing through the container. U.S. Pat. No. 2,092,190, issued Sept. 7, 1937, accomplishes essentially the same oscillatory motion with a can laid on its side in a horizontal plane. U.S. Pat. No. 2,109,233, issued Feb. 22, 1938, describes a mixing motion wherein the axis of the container moves along a straight line while at the same time the container ends circumscribe roughly elliptical paths in opposite directions. U.S. Pat. No. 2,797,902, issued July 2, 1957 discloses a mixing motion wherein the paint container is subjected to a combined lateral swinging movement and a simultaneous horizontal oscillatory movement in which the lateral swinging motion is accomplished on a pivotal axis located below the center of gravity of the container and its contents. U.S. Pat. No. 3,552,723, issued Jan. 5, 1971, discloses a mixing motion wherein a paint container is given an unequal rocking motion about a pivot point causing the paint to circulate in one direction within the can, the axis about which the rocking motion is imparted being generally horizontal. U.S. Pat. No. 3,880,408, issued Apr. 29, 1975, describes a device for mixing paints wherein a frame is rotatably attached to a pedestal to permit rotation about a first axis, and the frame supports a can holder which is rotatably movable about a second perpendicular axis, and the drive means to cause the can to rotate about the second axis at the same time as the frame is rotating about the first perpendicular axis. Finally, U.S. Pat. No. 3,542,344, issued Nov. 24, 1970, discloses a paint mixer wherein a vertically positioned can is first rapidly rotated about a first vertical axis through the can, suddenly stopped and reverse-rotated about the same axis, and the action is repeated, the intention being to provide an internal vortex in the paint liquid which vortex is developed, destroyed, and redeveloped in the opposite direction.
All of the foregoing patents describe empirically derived machines and methods for imparting violent agitation to liquid within a container in one way or another with the hoped-for end result of obtaining a good fluid mix. A good fluid mix is frequently especially difficult to obtain with paint, because the components tend to settle out and accumulate on the bottom of the can during the shelf life of the paint container. These components must be brought back into suspension in the liquid in order to provide a paint which has the proper color and consistency for coating applications. It has heretofore been thought that the mixing operation could be best accomplished by violent agitation of the container in most any direction or directions for some limited period of time.
It has been difficult to obtain theoretical data relating to the conditions of fluid agitation which occur within the paint container, for the motion therein is a complex turbulent motion which is theoretically difficult, if not impossible to describe. Most of the theoretical studies of fluid turbulence have dealt with fluid behavior in a moving closed container. For example, in a book entitled Boundary-Layer, Theory, by Dr. Hermann Schlichting, published by the McGraw-Hill BooK Company, 1968, the observation is made that velocity and pressure at a fixed point in space under turbulent motion conditions do not remain constant with time but perform very irregular fluctuations of high frequency. "Lumps" of fluid perform such fluctuations, and these "lumps" do not consist of single molecules as assumed in the kinetic theory of gases; they are macroscopic fluid balls of varying small size. Scientific observation has confirmed that such velocity and pressure fluctuations also involve certain bigger portions of fluid volume which have their own intrinsic motion superimposed on the main fluid motion. Such "fluid balls" or "lumps" assume variable sizes which continually agglomerate and disintegrate, and reform and this action has been used to attempt to determine the scale of turbulence within any given set of conditions. It is believed that this type of pressure-velocity fluctuation, when produced in a paint mixing apparatus, creates turbulent conditions within the paint container which most satisfactorily and rapidly provide an effective mix of the fluid within the container. Therefore, it is desirable to devise a method for mixing paint which induces the maximum apparent turbulence into the paint, and it is an object of the present invention to provide such a method.
It is a further object of the present invention to provide a method for mixing paint which can be implemented by an apparatus at low energy costs, for although the prior art has recognized that violent agitation can be readily obtained by the application of high energy forces to liquids, the present invention contemplates a method for effectively mixing wherein the steps to perform the method consume a minimum amount of energy.
It is yet another object of the present invention to disclose a method of mixing which may be implemented by simple mechanical motion and which, to the extent possible, takes advantage of the forces of nature to accomplish the desired end result, and in this regard the present invention recognizes and utilizes the force of gravity in the performance of the method.